1. Field of the Invention
This invention relates to a fluorophosphate optical glass having optical constants, i.e. a refractive index (nd) of 1.45 to 1.54 and Abbe number (.nu.d) of 75 to 90. The optical glass of the present invention, having a relatively large refractive index and large Abbe number, is useful as a lens for an optical instrument to obtain an image of good quality.
2. Description of the Prior Art
Introduction of fluorine into a glass composition is essential for obtaining an optical glass of low dispersion. Furthermore, it is also necessary to introduce a phosphate as a glass-forming material so as to stabilize the glass. Such an optical glass has been known as disclosed in Japanese Patent Publication No. 7430/1957. However, the fluorophosphate optical glass of the prior art cannot avoid fluctuation of the optical constants and striae due to vaporization of fluorine. In order to solve this problem, for example, an optical glass has been proposed which is capable of maintaining a lower vaporization of fluorine, as disclosed in Japanese Patent Publication No. 12857/1986.
In the known optical glasses, it has been proposed to introduce an alkali metal metaphosphate and melt at a low temperature for the purpose of improving the stability, for example, as in a glass composition comprising 8 to 11% of Al(PO.sub.3).sub.3, 0.5 to 5% of MPO.sub.3, 5 to 25% of BaF.sub.2, 10 to 35% of SrF.sub.2, 5 to 30% of CaF.sub.2, 5 to 20% of MgF.sub.2, 13 to 30% of AlF.sub.3 and 0 to 10% of LaF.sub.3, wherein M is at least one member selected from the group consisting of Li, Na and K and % is to be taken as that by mole, as disclosed in Japanese Patent Publication No. 12857/1986. However, the use of such an alkali metal metaphosphate results in contamination with more impurities such as water causing vaporization of fluorine than when using other metaphosphates. Thus, these optical glasses have problems in the production or processing thereof such that fluorine tends to vaporize so that fluctuation of the optical constants and striae takes place and the chemical durability is relatively low due to the chemical composition.